1. Field of the Invention
The present invention relates to methods and apparatus for reducing drill bit wear and more particularly to such a method and apparatus which reduces the wear on a turbine-driven drill bit being lowered into a wellbore.
2. Setting of the Invention
A common configuration for drilling wells includes a drill bit suspended from a string of drill pipe. Often, the drill bit includes openings in the lower end thereof to enable circulation of drilling fluid down the pipe string, through the bottom of the bit, and upwardly into the annulus between the outer surface of the pipe string and the wellbore. In conventional rotary drilling, the drill string is rotated while the fluid circulation flushes cuttings from the bottom of the wellbore and cools the drill bit.
In another form of drilling, a cylindrically-shaped downhole hydraulic motor is suspended from the lower end of a string of drill pipe and a drill bit having openings in the lower end thereof is mounted on a tubular drive shaft which extends from the lower end of the motor. Fluid is circulated down the drill string and through the motor thereby rotating the drive shaft. Fluid flow continues through the drive shaft, out the bottom of the bit and into the annulus. Thus, the fluid powers the downhole motor and, as in conventional rotary drilling, flushes cuttings from the bottom of the hole and cools the drill bit during drilling.
During the drilling of a well, it is usually necessary to periodically change the drill bit. The number of drill bits needed to complete the bore is dependent upon the hardness of the rock and the depth of the bore. When it is necessary to change the bit, the drill pipe is lifted out of the bore until the motor and bit are at the surface. A new bit is then mounted on the lower end of the motor and the string of drill pipe, with the motor and bit on the lower end thereof, is lowered into the wellbore.
When lowering a bit into a wellbore, it is sometimes desirable to rotate the bit to ream the bore. If the bit is not rotated and it should encounter an obstruction, e.g., a rock ledge or an undersize portion of the bore, the weight of the drill string on the bit could deform or otherwise damage the bit. In order to prevent such from occurring, the bit may be rotated thereby reaming the bore during downward travel.
One type of downhole motor which may be used to drive the drill bit is a turbine. The turbine experiences its maximum speed, which may be two or three thousand revolutions per minute or higher, under a no-load condition. Thus, when reaming a bore as described above, the turbine is likely to be rotating the drill bit at maximum speed. As the drill string is lowered, the bit is loaded only on its outer portion or gage area as the bore is reamed. If the gage area should strike a protruding rock ledge or the like under such conditions, the bit may be excessively worn or, in the worst case, broken.
It is not uncommon for the lower portion of the bore to be filled with dirt and rock rubble. Such rubble results from caving in of the bore walls during the trip out and from cuttings generated by reaming on the trip back in. Even if it is not necessary or desirable to ream a bore, it is important to begin circulation through the drill string above the rubble. Such circulation flushes the rubble upwardly in the annulus between the bore and the drill string and thus exposes the bottom of the bore for continued drilling. When circulation is started, the drill bit begins turning at a high rate of speed and may suffer damage or increased wear as described above.
There exists a need for a method and apparatus for reducing drill bit wear. Moreover, there exists a need for such a method and apparatus which reduces the wear on a turbine-driven drill bit while it is being lowered in a wellbore.